Dense gas in low-metallicity galaxies

Abstract

Stars form out of the densest parts of molecular clouds. Far-IR emission can be used to estimate the Star Formation Rate (SFR) and high dipole moment molecules, typically HCN, trace the dense gas. A strong correlation exists between HCN and Far-IR emission, with the ratio being nearly constant, over a large range of physical scales. A few recent observations have found HCN to be weak with respect to the Far-IR and CO in subsolar metallicity (low-Z) objects. We present observations of the Local Group galaxies M33, IC10, and NGC6822 with the IRAM 30meter and NRO 45m telescopes, greatly improving the sample of low-Z galaxies observed. HCN, HCO${}^{+}$, CS, C${}_2$H, and HNC have been detected. Compared to solar metallicity galaxies, the Nitrogen-bearing species are weak (HCN, HNC) or not detected (CN, HNCO, N${}_2$H${}^{+}$) relative to Far-IR or CO emission. HCO${}^{+}$ and C${}_2$H emission is normal with respect to CO and Far-IR. While ${}^{13}$CO is the usual factor 10 weaker than ${}^{12}$CO, C${}^{18}$O emission was not detected down to very low levels. Including earlier data, we find that the HCN/HCO${}^{+}$ ratio varies with metallicity (O/H) and attribute this to the sharply decreasing Nitrogen abundance. The dense gas fraction, traced by the HCN/CO and HCO${}^{+}$/CO ratios, follows the SFR but in the low-Z objects the HCO${}^{+}$ is much easier to measure. Combined with larger and smaller scale measurements, the HCO${}^{+}$ line appears to be an excellent tracer of dense gas and varies linearly with the SFR for both low and high metallicities.

9 pages, 13 figures, A&A in press

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• The ALMOND Survey: Molecular cloud properties and gas density tracers across 25 nearby spiral galaxies with ALMA
• A molecular line survey toward the nearby galaxies NGC 1068, NGC 253, and IC 342 at 3 mm with the Nobeyama 45 m radio telescope: The data
• An ALMA Glimpse of Dense Molecular Filaments Associated with High-mass Protostellar Systems in the Large Magellanic Cloud
• The Evolution of NGC 7465 as Revealed by its Molecular Gas Properties
• Tracers of Dense Gas in the Outer Galaxy
• Dense Gas and Star Formation in Nearby Infrared Bright Galaxies: APEX survey of HCN and HCO+ J=2-1
• Weak CS Emission in an Extremely Metal-poor Galaxy DDO 70